The present invention relates to a polycarbonate, more particularly to a polycarbonate having excellent impact resistance at low temperature and a resin composition containing the said polycarbonate.
Polycarbonates are widely used in many fields of industries as a resin with many advantageous properties such as high heat resistance, impact resistance and transparency. As to a method for producing polycarbonates, an interfacial method comprising reacting directly an aromatic dihydroxy compound such as bisphenol A with phosgene is generally used. However, in the interfacial method, there arise the problems such as poor thermal stability of the said polycarbonate and the corrosion of the mold at the time of injection molding caused by remaining methylene chloride in the polycarbonate. Therefore, a melting method comprising reacting an aromatic dihydroxy compound with a carbonic acid diester has been evaluated again because in addition to the advantages that there are no problems mentioned above in the interfacial method, polycarbonates can be produced by the melting method more inexpensive than those by the interfacial method.
However, comparing with the polycarbonate produced by the interfacial method, the polycarbonate produced by the melting method tends to reduce impact strength at low temperature, so that the polycarbonate is limited in practical use in which the impact strength at low temperature is especially required, such as automobile parts and OA equipment parts. As a method for improving the impact strength at low temperature of polycarbonate produced by the melting method, there are generally used a method of increasing the molecular weight and a method in which various elastomers are added into the polycarbonate. However, the molded article obtained by these methods has problems that flowabilty of polycarbonate is poor, color tone thereof is deteriorated and transparency thereof is deteriorated as the case.
An object of the present invention is to provide a polycarbonate having excellent impact resistance at low temperature and a resin composition containing the said polycarbonate.
As a result of the present inventor is earnest studies to attain the above object, it has been found that a polycarbonate obtained by a melting method and having a specific molecular weight and a specific melt viscoelasticity has better impact strength at low temperature than the other types of polycarbonate having the substantially same molecular weight. The present invention has been attained on the basis of the above finding.
Thus, in the first aspect of the present invention, there is provided a polycarbonate which can be obtained by a melting method and has a viscosity-average molecular weight of 16000 to 27000, and whose loss angle xcex4 and complex viscosity xcex7* (Paxc2x7s) measured under the conditions of 250xc2x0 C. and an angular velocity of 10 rad/s satisfy the following relation (1):
4,700xe2x89xa6Tan xcex4/(xcex7*)xe2x88x920.87xe2x80x83xe2x80x83(1) 
In the second aspect of the present invention, there is provided a polycarbonate resin composition comprising a polycarbonate (as defined in the first aspect) which can be obtained by a melting method and has a viscosity-average molecular weight of 16000 to 27000, and whose loss angle xcex4 and complex viscosity xcex7* (Paxc2x7s) measured under the conditions of 250xc2x0 C. and an angular velocity of 10 rad/s satisfy the following relation (1):
4,700xe2x89xa6Tan xcex4/(xcex7*)xe2x88x920.87xe2x80x83xe2x80x83(1)